My question is about turbofan airliners, could an airplane tilt its wings 90 degrees so that the fuselage horizontal axis is perpendicular to the longitudinal axis of the turbo fan. This way an airliner could be VTOL and at the same time it could have even 90 degrees angle of attack. Am I missing something?
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$\begingroup$ I am not much understanding, do you want it to tilt its angle at take and landing, otherwise the wings are normal? $\endgroup$– user40476Jun 13, 2019 at 17:54
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$\begingroup$ I could have sworn we've had this question before, but I can't find it atm. $\endgroup$– AEhere supports MonicaJun 13, 2019 at 17:56
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$\begingroup$ Before asking if its possible, ask if its even desirable? What possible use could there be for a VTOL airliner? They're nice for certain military applications, but just pointless for civilian passengers. You consider the additional expense, fuel, weight, and complexity, and realize its a solution searching for a problem. $\endgroup$– abelenkyJun 13, 2019 at 18:34
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4$\begingroup$ @abelenky The user is asking if it's possible. You might not find that an interesting question, or even one worth asking, but it's what the user would like to know. The user doesn't need to read your obvious disdain for his or her naïvety. $\endgroup$– Daniele ProcidaJun 13, 2019 at 19:50
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4$\begingroup$ The answer to any "is it possible" question is virtually always, "yes, with enough time, money and resources, virtually anything is possible". Possible is not a reasonable criteria. $\endgroup$– abelenkyJun 13, 2019 at 23:12
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3 Answers
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What you describe is called a tiltwing. It has wing-mounted engines which swivel with the wing so their thrust points up for the VTOL mode. Building a tiltwing airliner will face two challenges:
- Normally, the static thrust force is only one third of the weight force, so the tiltwing airliner needs to add three times as many engines - at least. Better make this four times as many so there is some excess thrust for hot and high conditions.
- In order for the wing to tilt freely, it needs to be straight. A swept wing will hit the ground during tilting unless an absurdly high landing gear is installed. Now the thrust acts along one line - how do you control pitch? You will need another engine at the tail (or nose) which can be quickly throttled in order to balance the aircraft in pitch.
Needless to say that this cuts into payload and makes this design uncompetitive unless VTOL operation is a must.
answered Jun 13, 2019 at 17:59
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Could? Yes.
Would it make sense and be viable? No, it massively complicates the structure, fuel system, and hydraulics. It also requires a different FCS for the transition and hover flight, as well as very heavy actuators to actually tilt anything.
The closest thing that exists nowadays are tilt-rotors like the V-22, which only tilt the engines. Other proposals in the same area go further and tilt only the rotors themselves, precisely to avoid turning a huge part of the airframe.
answered Jun 13, 2019 at 17:59
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$\begingroup$ You also need engines far more powerful than if you used a runway, and add a stability system to control the airplane at low speed. More weight and complexity. $\endgroup$– GdDJun 13, 2019 at 19:37
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I addition to what Peter Kämpf has written, regarding point 1. If I unterstand the things correctly a plane needs generally a thrust-to-weight-ratio above 1.0 to be able to fly vertical. Planes are generally below 1.0 because their wings generate enough lift during flight and more powerful engines would be ineffective. For example the 747 is around 0.27 and as Peter Kämpf noted, VTOL capability would require 12 additional engines.
https://en.wikipedia.org/wiki/Thrust-to-weight_ratio
Some fighter jets have more than 1.0 which are, for example, the Typhoon or the F-15. They shall be superior over others and so efficiency is defined different. And of course also the Harrier, not designed as superior fighter, but it must operate as VTOL aircraft. While not listed, I think the V-22 also must be above 1.0 ratio. Interestingly, the F-35 is with full fuell tanks far below 1.0 but it is a STOVL aircraft, not VTOL.
